Parsers raise Noparse to indicate that they were not able to make any
progress at all. If p is such a parser, fix s p gives a new parser where
a Noparse exception from p will result in a Failure s exception, but is
otherwise the same as p.

FAILURE CONDITIONS

The immediate call fix s p never fails, but the resulting parser may.

COMMENTS

This is one of a suite of combinators for manipulating ``parsers''. A parser is
simply a function whose OCaml type is some instance of
:('a)list -> 'b * ('a)list. The function should take a list of objects of
type :'a (e.g. characters or tokens), parse as much of it as possible from
left to right, and return a pair consisting of the object derived from parsing
(e.g. a term or a special syntax tree) and the list of elements that were not
processed.